1. Field of the Invention
The present invention relates to an electric motor having one or more rotation sensors, each of which outputs a signal that corresponds to a rotational speed of an output shaft of the motor. The present invention further relates to a manufacturing method of such a motor.
2. Description of Related Art
In one previously proposed outer rotor motor, rotation sensors and a circuit board, which includes a control circuit, are secured to a stator. Each rotation sensor outputs a signal that corresponds to a rotational speed of an output shaft of the outer rotor motor. The control circuit controls the rotational speed of the output shaft, i.e., a pattern of power supply (hereinafter referred to as a power supply pattern) to coils of the stator based on the output signals of the rotation sensors.
One such an outer rotor motor will be described with reference to FIGS. 21A and 21B. FIG. 21A shows a partially fragmented front view of the outer rotor motor 200. FIG. 21 shows a cross sectional view of the outer rotor motor 200. The outer rotor motor 200 includes a stator, which has a stator base 202. The stator base 202 includes a tubular portion 204 and an extension 206. The extension 206 extends radially outwardly from a base end of the tubular portion 204 and has a square shape when the extension 206 is viewed in a direction perpendicular to a plane of the extension 206. A stator core 209, around which coils 208 are wound, is secured to an outer peripheral portion of the tubular portion 204.
Furthermore, an output shaft 210 is rotatably received in the tubular portion 204 through bearings 212. A rotor housing (yoke) 214 is coaxially secured to one end of the output shaft 210. Furthermore, magnets 216 are secured to an inner peripheral surface of the rotor housing 214, which surrounds a radially outer part of the stator core 209. With the above structure, when electrical power is supplied to the coils 208, the rotor housing 214 and the output shaft 201 are rotated together by magnetic force of the coils 208 and magnetic force of the magnets 216.
A sensor plate 218, which has a circular disc shape, is coaxially secured to the output shaft 210 at axially outward of the extension 206. The sensor plate 218 rotates with the output shaft 210. An outer peripheral portion of the sensor plate 218 includes a predetermined number of slits 218A, which are arranged at equal intervals in a circumferential direction of the sensor plate 218.
Two rotation sensors 220, 222 are arranged in a symmetrical manner with respect to the rotational axis of the sensor plate 218. Each rotation sensor 220, 222 has two opposed arms, so that the rotation sensor 220, 222 has a horseshoe-shaped cross section. A slit forming portion of the sensor plate 218, which has the slits 218A, is received between the arms of each rotation sensor 220, 222. A light emitting element is provided to one of the arms of each rotation sensor 220, 222, and a light receiving element is provided to the other one of the arms of the rotation sensor 220, 222. An on/off pulse signal is outputted depending on whether light emitted from the light emitting element is received by the light receiving element through a corresponding slit 218A. The outputted on/off signal (pulse width) corresponds to the rotational speed of the sensor plate 218.
The rotation sensor 220 is installed to a circuit board 224 that is secured to a circuit board holding portion 206A, which is recessed in the extension 206. The rotation sensor 222 is installed to a circuit board 226 that is secured to a circuit board holding portion 206B, which is recessed in the extension 206, separately from the circuit board 224. Each circuit board 224, 226, which includes the corresponding rotation sensor 220, 222, is slid relative to the corresponding circuit board holding portion 206A, 206B while a distal end of the rotation sensor 220, 222 is oriented toward the rotational axis of the sensor plate 218. Thus, the sensor plate 218 is received between the arms of the corresponding rotation sensor 220, 222. Each circuit board 224, 226 includes a connector 224A, 226A, through which output signals of the corresponding rotation sensor 220, 222 are outputted to a control circuit board 228.
The control circuit board 228 is formed into a square shape, which corresponds to the shape of the extension 206. The tubular portion 204 is received through a through hole 228A of the control circuit board 228, which is formed in a center of the control circuit board 228. The control circuit board 228 is secured to a stator core 209 side of the extension 206. This arrangement of the control circuit board 228 allows effective installation of a power supply circuit, which supplies electrical power to the coils 208, to the control circuit board 228 and also allows effective installation of a Hall element 230, which measures magnetic pole positions of the magnets 216, to the control circuit board 228.
The control circuit board 228 further includes a control circuit. The control circuit maintains the rotational speed of the sensor plate 218. i.e., the rotational speed of the output shaft 210 at a preset target speed by changing the power supply pattern to the coils 208 when an average value of rotational speed measurements of the rotation sensors 220, 222 obtained based on the output signals of the rotation sensors 220, 222 differs from the preset target speed. Thus, a portion of the circuit board 228, which extends outwardly from the extension 206, has a connector 232 that is connected to the connectors 224A, 226A and to the power supply source through connectorized cables (not shown).
Furthermore, the extension 206 has a cover 234, which covers the sensor plate 218 and the rotation sensors 220, 222 to limit intrusion of light and foreign particles or objects. With this arrangement, reading errors from the rotation sensors 220, 222 can be limited.
However, in the above outer rotor motor 200, the control circuit board 228, the circuit board 224 having the rotation sensor 220, and the circuit board 226 having the rotation sensor 222 are separately provided. Thus, the number of components and the number of assembling steps are disadvantageously increased, resulting in an increase in manufacturing costs. Furthermore, each circuit board 224, 226, 228 needs to have the connector 224A, 226A, 232, and electrical connections between the circuit boards 224, 226, 228. Thus, the number of the components and the number of the assembling steps are disadvantageously increased. Particularly, in the case of the outer rotor motor 200, which includes the two rotation sensors 220, 222, the above disadvantages need to be considered.
In attempts to integrate the circuit boards 224, 226, 228 into a single circuit board having, for example, the control circuit and the rotation sensor 220, it is difficult to have a sufficient circuit board surface area for installing the control circuit while limiting a size of an overhang from the extension 206 (rotor housing 214), and also it is difficult to insert the sensor plate 218 between the two arms (i.e., portions that limit axial movement of the sensor plate) of, for example, the rotation sensor 220 installed in the circuit board.
Besides the above outer rotor motor 200, there are other previously proposed outer rotor motors. For example, Japanese Unexamined patent Publication No. 2000-346144 does not disclose a circuit board, to which a rotation sensor is installed, but discloses the rotation sensor and a control circuit board, which are separately provided. The separation of the rotation sensor and the control circuit board is similar to the outer rotor motor 200, and thereby the number of the components is still large. Thus, it is difficult to reduce manufacturing costs. Japanese Unexamined Patent Publication No. 2002-78290 discloses a motor but does not disclose a control circuit board provided in the motor. However, in Japanese Unexamined Patent Publication No. 2002-78290, a rotation sensor is independent from a controller. This feature is similar to the outer rotor motor 200 and thus results in the relatively large number of the components and the relatively high manufacturing costs. Japanese Unexamined Patent Publication No. 11-122887 discloses a rotation sensor unit, which is installed to a motor in a later process. This also results in a relatively large number of the components and the relatively high manufacturing costs.